Rapid visual detection of glutathione in vegetables and distinguishing multiple sulfur-containing compounds by smartphone-assisted sensor based on calcined PPy@CuNi-ZIF-67 nanozyme with enhanced oxidase-like activity

In this work, polypyrrole-coated Cu, Ni co-doped ZIF-67 (PPy@CuNi-ZIF-67) was designed and prepared via a facile method. Calcined PPy@CuNi-ZIF-67 exhibited highly efficient oxidase-like activity and possessed excellent capacity to oxidize colorless 3,3′,5,5′-tetramethyl benzidine (TMB) into blue oxidation products (oxTMB) within 4 min. Glutathione (GSH) and sulfur-containing compounds can inhibit its oxidase-like activity. Based on this principle, an effective colorimetric method was developed in combination with a smartphone for efficient, sensitive, and visual detection of GSH. Under optimal conditions, good linearity was achieved in the range of 0.83–33.33 μM, and the visual detection limit was 0.27 μM. Moreover, based on the response difference of different sulfur-containing compounds to different chromogenic substrates, a new array sensor was constructed with three chromogenic substrates. The array sensor was proven to quickly distinguish five sulfur-containing compounds at a concentration as low as 8.3 μM in combination with principal component analysis and hierarchical clustering analysis.